A radome can be referred to as an electromagnetic window or radiation dome and is usually used in automobiles or in stationary operation and in order to protect antennas, for example, from external influences, particularly environmental influences. One requirement placed on a radome is that it be electromagnetically permeable (transparent) in order to allow electromagnetic waves to pass through that are transmitted by an antenna or are to be received by the antenna.
In the case of flying platforms camouflaged against radar detection, particularly the section of the platform that is located behind a radome can be a source of reflection of the radar.
The radar-camouflaging of flying platforms is achieved, among other things, through the shape of an electrically conductive outer skin of the platforms, thus influencing the reradiation of electromagnetic waves from this outer skin to a radar. Such platforms can be referred to as platforms with reduced radar signature. A neuralgic point of high reradiation can lie particularly in the antennas of the radar and communication systems of flying platforms, provided that a radome of the antenna apertures is designed with aerodynamic and not camouflage-related aspects in mind. Since a radome is usually designed so as to be electromagnetically permeable in order to permit the transmission of signals from and to the radar and communication systems, a radome is also permeable to radar signals of a detecting radar. Strongly back-scattering antenna apertures can thus weaken or inadmissibly impair the camouflaging of the flying platform.
Known radomes can have a frequency-selective layer which allows signals of its own radar and communication systems to pass through and reflects signals of other frequencies in a desired direction or does not allow them to pass through. The drawback of this solution is that a camouflage effect is not provided for the working frequency of one's own radar and communication systems.
One broadband approach to a solution is a radome that is permeable to all frequencies in a first state and impermeable or reflective to all frequencies in a second state.
One example of such a radome that is optionally permeable or reflective to electromagnetic waves is a radome with a photosensitive layer, as described in DE 39 20 110 A1. The photosensitive layer consists of a semiconductor photoresistor. In this material, incident photons bring about a release of charge carriers in the material of the semiconductor. The conductivity of the semiconductor is thus dependent on an incident illumination level and can be reversibly changed, whereby the permeability of the semiconductor and hence of the radome to electromagnetic waves can be controlled. In a low charge carrier state (unilluminated), the layer is electromagnetically permeable, whereas it is electromagnetically impermeable in a state with loose or released charge carriers (illuminated). The conductivity of the semiconductor elements can change by several orders of magnitude between the unilluminated and the illuminated state, for example by up to 4 to 5 powers of ten.
Due to the platform-specific shape of a radome, the physical characteristics of usable radome materials and the requirements placed on the photosensitive coating, it is a technical challenge to provide a radome with a suitable photosensitive semiconductor coating.